Tektites are a rare type of impact glass; they are found in only four distinct and geographically extended strewn fields, ranging from 0.8 to 35 million years in age. For three of these four strewn fields, the source crater is known. Microtektites from three of the four fields are also known from deep-sea cores, in addition to normal, centimeter-sized specimens; their distribution defines the extent of the strewn fields. Tektite-like glasses have more recently been found in a few other locations, but they show limited distribution, or few specimens have been recovered, or their sources and origins are not well understood, or a combination of these circumstances may be the case. Tektites differ from “normal” impact glasses in that they were derived from the very surface of the target area (as is indicated by their high content of the cosmogenic radioisotope 10Be) and may have formed and been ejected before the main crater excavation phase even began. They have very minor meteoritic components, as indicated in, for example, Os isotopic studies. As most tektites are homogeneous glass, they must have experienced extremely high formation temperatures. Twenty-first-century work has shown that Zn, Cu, Cd, and Sn are isotopically fractionated by volatilization in tektites. The study of unconventional stable isotopes provides interesting clues regarding the formation, differentiation, and deposition of tektites. Tektites are clearly an interesting and unusual subtype of impact glasses.
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Tektites
Christian Koeberl
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Terrestrial Planets: Interior Structure, Dynamics, and Evolution
Doris Breuer and Tilman Spohn
The three terrestrial planets Mercury, Venus, and Mars (ordered by their distance from the sun) share the same first-order internal structure with the Earth. There is an iron-rich core at the center, overlain by a silicate mantle and a crust that is generated by partial melting of the mantle. But while Mars and Venus have a core with a radius of about half the planetary radius, just as the Earth, the core of Mercury extends to about 80% of the planet’s radius. The interiors of the terrestrial planets are heated by the decay of radioactive elements and cool by removing internal energy. In addition to radiogenic heat, internal energy was deposited during planet formation and early differentiation. Heat transport is dominated by mantle and core convection and volcanic heat transfer although conduction through the lithosphere on top of the mantle matters. The convection powers the planetary heat engine which converts thermal energy into gravitational energy, mechanical (tectonic) work, and magnetic field energy. None of the terrestrial planets has plate tectonics such as the Earth although surface renewal and some form of lithosphere subduction is debated for Venus. The tectonics of Mars and Mercury is best described as stagnant-lid tectonics, with a thick rigid lid overlying the convecting mantle. Both planets show early volcanism, with Mars in particular being locally volcanically active even until a few million years ago. Because of Mercury’s large core, the mantle is comparatively thin, and convection may be sluggish or may even have ceased. Magnetism is another property that the terrestrial planets share with the Earth although it is still not confirmed by data that Venus ever had a magnetic field. A dynamo process driven by buoyancy released through the growth of a solid inner core is producing the present-day magnetic fields of Earth and Mercury, but Mars’ dynamo has likely ceased to be active. Crust units with remanent magnetization testify to the early dynamo. The terrestrial planets have been explored to differing degrees by spacecraft missions which allow a deeper physical understanding of the interiors and their dynamics and evolution.
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Landslides in the Solar System
Maria Teresa Brunetti and Silvia Peruccacci
Landslides are mass movements of rock, earth, or debris. All of these surface processes occur under the influence of gravity, meaning that they globally move material from higher to lower places. On planets other than Earth, these structures were first observed in a lunar crater during the Apollo program, but mass movements have been found on many rocky worlds (solid bodies) in the Solar System, including icy satellites, asteroids, and comets.
On Earth, landslides have the effect of shaping the landscape more or less rapidly, leaving a signature that is recognized through field surveys and visual analysis or automatic identification on ground-based, aerial, and satellite images.
Landslides observed on Earth and on solid bodies of the Solar System can be classified into different types based on their movement and the material involved in the failure. Material is either rock or soil (or both), with a variable fraction of water or ice; a soil mainly composed of sand-sized or finer particles is referred as earth while debris is composed of coarser fragments. The landslide mass may be displaced in several types of movement, classified generically as falling, toppling, sliding, spreading, or flowing. Such diverse characteristics mean that the size of a landslide (e.g., area, volume, fall height, length) can vary widely. For example, on Earth, their area ranges up to 11 orders of magnitude, while their volume varies by 16 orders, from small rock fragments to huge submarine landslides.
The classification of extraterrestrial landslides is based on terrestrial analogs having similarities and characteristics that resemble those found on planetary bodies, such as Mars. The morphological classification is made regardless of the geomorphological environment or processes that may have triggered the slope failure.
Comparing landslide characteristics on various planetary bodies helps to understand the effect of surface gravity on landslide initiation and propagation—of tremendous importance when designing manned and unmanned missions with landings on extraterrestrial bodies.
Regardless of the practical applications of such study, knowing the morphology and surface dynamics that shape solid bodies in the space surrounding the Earth is something that has fascinated the human imagination since the time of Galileo.
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Large Volcanic Channels of the Inner Solar System
David W. Leverington
Many large volcanic channel systems are recognized at the surfaces of rocky bodies of the inner solar system. The more than 200 channels known for the Moon mainly have simple sinuous forms with widths of up to several kilometers and lengths of up to hundreds of kilometers, typically commencing at topographic depressions and extending downslope until they fade into associated volcanic units. The Rima Hadley system was a key target of the Apollo 15 mission and was confirmed as a product of volcanic processes related to the emplacement of lavas in the Palus Putredinis region of Mare Imbrium. The more than 200 channels known for Venus are in many cases morphologically similar to sinuous lunar rilles, but some systems are especially large and complex, with widths of up to tens of kilometers and lengths that can exceed 1,000 km. Such systems typically commence at structural features or in regions of disturbed terrain and possess anastomosing reaches associated with prominent streamlined uplands. In contrast, Venusian canali typically maintain sinuous forms with widths of only a few kilometers but remarkably can be characterized by lengths of thousands of kilometers. Some Venusian channels were involved in the emplacement of fluidized ejecta in the vicinities of impact craters whereas others may have formed in such environments as a result of later volcanic events. The 10 large volcanic channels that are recognized on Mercury have lengths no greater than ~161 km but can have widths of up to several tens of kilometers. These systems developed as conduits for voluminous lavas that extend across adjacent impact basins. Terrestrial komatiitic channels of Archean and Proterozoic ages can have sizes that are comparable to those of lunar rilles, and the formation of these systems is likely to have played an important role in the development of associated Ni-Cu-(PGE) ores. The outflow channels of Mars have widths of up to tens of kilometers and lengths of up to thousands of kilometers and are widely interpreted as aqueous systems formed by catastrophic discharges from aquifers, but the properties and geological associations of these features and numerous other large Martian channel systems are arguably well aligned with those expected of volcanic origins. Overall, large volcanic channels of the inner solar system are mainly ancient products of the emplacement of low-viscosity lava flows of mafic or ultramafic composition, involving eruptions that were characterized by extraordinarily high effusion rates and total lava volumes that in some cases are likely to have been as great as those that characterized some Large Igneous Provinces on Earth. The deeply rooted igneous plumbing systems most favorable to the development of large volcanic channels would have been especially common in the earlier history of the solar system, when the interior temperatures of rocky bodies were greater than today. The early development of large volcanic channel systems is likely typical of the geological histories of large rocky bodies in the universe.
Article
The Qaidam Basin as a Planetary Analog
Jiannan Zhao, Yutong Shi, and Long Xiao
Analog study is a convenient and effective way to understand the geomorphic features and geological processes of other planets. The Qaidam Basin, an intramontane basin in the northeastern Tibetan Plateau, northwest China, is a new and unique Mars analog study site. The basin hosts the highest and one of the driest deserts on the Earth, and its environment is characterized as cold, arid, of high altitude, of high UV radiation, and of high soil salinity. A variety of landforms that are comparable to those on the Martian surface have been identified, such as dunes, yardangs, valleys, gullies, lakes, and playas, providing opportunities to study the formation and evolution of similar Martian geomorphic features. Aqueous minerals including chlorides, sulfates, carbonates, and phyllosilicates are concentrated in the saline lakes and playas of the basin. Analog studies on the mineral assemblages of the Qaidam playas and Martian paleolakes and playas will help researchers better understand the hydrological environment and climate of the ancient Mars. The extreme environment of the Qaidam Basin also makes it an ideal site for astrobiological study. Detection of biomarkers and the isolation of microorganisms in the basin could provide clues for the search for life and a habitable environment on Mars. In addition, the accessibility of the Qaidam Basin makes the basin a potential testing ground for instruments and study methods to be used in future Mars missions.
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Planetary Spectroscopy
Alian Wang
Planetary spectroscopy uses physical methods to study the chemical properties of the geological materials on the planetary bodies in our solar system. This article will present twelve types of spectroscopy frequently used in planetary explorations. Their energy (or wavelength) varies from γ-ray (keV) to far-infrared (μm), which involves the transitions of nuclei, atoms, ions, and molecules in planetary materials. The article will cover the basic concept of the transition for each of the twelve types of spectroscopy, along with their legendary science discoveries made during the past planetary exploration missions by the international planetary science and engineering community.
The broad application of spectroscopy in planetary exploration is built upon the fact that only limited extraterrestrial materials were collected (meteorites, cosmic dust, and the returned samples by missions) that enabled the detailed investigations of their properties in laboratories, while spectroscopic measurements can be made on the objects of our solar system remotely and robotically, such as during the flyby, orbiting, lander, and rover missions. In this sense, the knowledge obtained by planetary spectroscopy has contributed to a major portion of planetary sciences.
In the coming era of space explorations, more powerful spacecraft will be sent out by mankind, go to deep space, and explore exotic places. Generations of new planetary science payloads, including planetary spectrometers, will be created and will fly. New sciences will be revealed.
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Martian Dust
Steven W. Ruff
Dust makes the red planet red. Without dust, Mars would appear mostly as shades of gray. The reddish hue arises from a small amount of oxidized iron among its basaltic mineral constituents. In this sense, Mars is a rusty world. Martian dust is a ubiquitous material of remarkably uniform composition that spans the globe, filling the skies and covering the land in a temporally and spatially varying manner. It is routinely lifted into the atmosphere via convective vortices known as dust devils. Dust in the atmosphere waxes and wanes according to season. Every few Martian years, the planet is fully encircled in atmospheric dust of sufficient opacity that its surface markings and landforms are completely obscured from view of Earth-bound telescopes and Mars-orbiting satellites. Such global dust events last for weeks or months, long enough to jeopardize solar-powered spacecraft on the surface. Dust particles suspended in the thin Martian atmosphere ultimately fall to the surface, completing the cycle and contributing to a range of features that are still being discovered and investigated.
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Terrestrial Analogs to Planetary Volcanic Phenomena
Peter J. Mouginis-Mark and Lionel Wilson
More than 50 years of solar system exploration have revealed the great diversity of volcanic landscapes beyond Earth, be they formed by molten rock, liquid water, or other volatile species. Classic examples of giant shield volcanoes, solidified lava flows, extensive ash deposits, and volcanic vents can all be identified, but except for eruptions seen on the Jovian moon Io, no planetary volcanoes have been observed in eruption. Consequently, the details of the processes that created these landscapes must be inferred from the available spacecraft data. Despite the increasing improvement in the spatial, temporal, compositional, and topographic characteristics of the data for planetary volcanoes, details of the way they formed are not clear. However, terrestrial eruptions can provide numerous insights into planetary eruptions, whether they are effusive eruptions resulting in the emplacement of lava flows or explosive eruptions due to either volatiles in the magma or the interaction between hot lava and water or ice. In recent decades, growing attention has been placed on the use of terrestrial analogs to help interpret volcanic landforms and processes on the rocky planets (Mercury, Venus, the Moon, and Mars) and in the outer solar system (the moons of Jupiter and Saturn, and the larger asteroids). In addition, terrestrial analogs not only provide insights into the geologic processes associated with volcanism but also can serve as test sites for the development of instrumentation to be sent to other worlds, as well as provide a training ground for crewed and uncrewed missions seeking to better understand volcanism throughout the solar system.
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Water Ice Permafrost on Mars and on the Moon
Maxim Litvak and Anton Sanin
The Moon and Mars are the most explored planetary bodies in the solar system. For the more than 60 years of the space era, dozens of science robotic missions have explored the Moon and Mars. The primary scientific goal for many of these missions was declared to be a search for surface or ground water/water ice and gaining an understanding of its distribution and origin.
Today, for the Moon, the focus of scientific exploration has moved to the lunar polar regions and permanently shadowed regions (PSRs). PSRs do not receive any direct sunlight and are frozen at very low temperatures (< 120 K), acting as cold traps. They are considered to be a storehouse that preserves records of the solar system’s evolution by trapping water ice and potentially other volatile deposits brought by comets and asteroids over billions of years.
For Mars, the water/water ice search was part of an attempt to find traces of ancient extraterrestrial life and possibly to understand how life appeared on Earth. Current Mars is cold and dry, but its high latitudes and some equatorial regions are enriched with surface and subsurface water ice. Scientists argue that oceans could have existed on ancient Mars if it was warm and wet and that different life forms could have originated similar to Earth’s. If this is the case, then biomarkers could be preserved in the Martian ground ice depositions.
Another popular idea that ties water ice permafrost on the Moon and Mars is related to the expected future human expansion to deep space. The Moon and Mars are widely considered to be the first destinations for future manned space-colony missions or even space-colony missions. In this scenario, the long-term presence and survival of astronauts on the lunar or Martian surface strongly depend on in situ resource utilization (ISRU). Water ice is at the top of the ISRU list because it could be used as water for astronauts’ needs. Its constituents, oxygen and hydrogen, could be used for breathing and for rocket fuel production, respectively.
The Moon is the closest body to Earth and discussion about presence of water ice on the Moon has both scientific and practical interest, especially for planning manned space missions. The focus further in space is on how subsurface water ice is distributed on Mars. A related topic is the debates about whether ancient Mars was wet and warm or if, for most of its history, the Martian surface was covered with glaciers. Finally, there are fundamental questions that should be answered by upcoming Mars and Moon missions.
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Impact Crater Densities as a Tool for Dating Planetary Surfaces
William K. Hartmann
The use of impact crater densities to estimate the ages of planetary surfaces began in the 1960s. Some predictive successes have been confirmed with radiometric dating of sites on the Moon and Mars. The method is highly dependent on our understanding of the rate of crater formation on different worlds, and, more importantly, on the history of that rate, starting with intense cratering during planetary formation 4.5 Ga ago. The system is thus calibrated by obtaining radiometric dates from samples of relatively homogeneous geologic units on various worlds.
Crater chronometry is still in its infancy. Future sample-returns and in situ measurements, obtained by missions from collaborating nations to various worlds, will provide ever-increasing improvements in the system in coming decades. Such data can lead to at least two-significant-figure measurements, not only of the ages of broad geologic provinces on solar system worlds, but of the characteristic survival times of various-sized smaller craters. Such data, in turn, clarify the rates of turnover of surface materials and the production rates of gravel-like regolith and megaregolith in the surface layers. Better measurements of the impact rate at various times, in turn, support better modeling of the accretion and fragmentation processes among early planetesimals as well as contemporary asteroids, in various parts of the solar system. Once the crater chronometry system is calibrated for various planetary bodies, important chronological information about those various planetary bodies can be obtained by orbital missions, without the need for expensive sample-return or lander missions on each individual surface.
